LMS Adaptive Filter for Digital Cancellation of Second Order Inter-Modulation Due to Transmitter Leakage

ABSTRACT

A transmit signal second-order inter-modulation (IM2) canceller for a portable handset using a full duplex mode of operation (e.g., WCDMA) is used to controllably reduce IM2 introduced by a transmit signal that appears in a received signal in a receive channel of the portable handset. The transmit signal IM2 canceller includes a delay estimator and a digital signal adjuster. The delay estimator receives a first input from a receive channel and a second input from a transmit channel. The delay estimator generates an estimate of the IM2 that the transmit channel introduces in the receive channel. The digital signal adjuster removes the estimate of the IM2 before forwarding a modified receive channel signal to a baseband subsystem of the portable handset.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending and commonlyassigned U.S. patent application Ser. No. 12/052,991, filed on Mar. 21,2008, entitled “LMS Adaptive Filter for Digital Cancellation of SecondOrder Inter-Modulation Due to Transmitter Leakage,” which is herebyincorporated into this document by reference in its entirety, and whichclaims priority to U.S. Provisional Patent Application titled, “LMSAdaptive Filter for Digital Cancellation of IM2 Due to TransmitterLeakage,” having application Ser. No. 60/896,019, filed on Mar. 21,2007, which is also hereby incorporated into this document by referencein its entirety.

BACKGROUND

This invention relates generally to transceiver architecture in awireless portable communication device.

With the increasing availability of efficient, low cost electronicmodules, mobile communication systems are becoming more and morewidespread. For example, there are many variations of communicationschemes in which various frequencies, transmission schemes, modulationtechniques and communication protocols are used to provide two-way voiceand data communications in a handheld, telephone-like communicationhandset. The different modulation and transmission schemes each haveadvantages and disadvantages.

In a 3G application for a system operating in the wideband code divisionmultiple access (WCDMA) communication system, a portable transceiveroperates in full-duplex mode. That is, both a receiver and a transmitterare operating simultaneously. In this operational mode, energy from thetransmitted signal generated in the handset “leaks” into the receiverchannel and generates a second-order inter-modulation signal component,which unfortunately falls in the same range of frequencies as thereceived signal.

Conventional approaches to reduce the transmitter signal leakage intothe receive channel include the introduction of one or moresurface-acoustic wave (SAW) filters. The transduction from electricenergy to mechanical energy (in the form of SAWs) is accomplished by theuse of piezoelectric materials. Electronic devices employing a SAWnormally utilize one or more interdigital transducers (IDTs) to convertan acoustic wave to an electrical signal and vice versa using thepiezoelectric effect of certain materials (e.g., quartz, lithiumniobate, lithium tantalate, lanthanum gallium silicate, etc.). Thesedevices are fabricated utilizing photolithography, the process used inthe manufacture of silicon integrated circuits.

SAW filters have been successfully applied in many cellular telephonearchitectures and provide significant advantages in performance, cost,and size over other filter technologies (e.g., digital signalprocessors, quartz crystals (bulk wave), LC filters, and waveguidefilters). The continued drive in the industry toward reducing cost anddevice size, as well as the desire to realize increased efficiencies,makes it desirable to remove SAW filters from the telephone. However,removal of a SAW filter before a duplexer that couples both a receivechannel and a transmit channel to a common antenna reintroduces theabove-described interference in a desired receive signal due to secondorder inter-modulation signal components from the transmit signal.

FIG. 1 is a schematic diagram illustrating the introduction oftransmitter generated second order inter-modulation (IM2) in a desiredreceive signal of a conventional full duplex transceiver. Thetransceiver includes a transmit channel upconverter or TX upconverter10, power amplifier 20, duplexer 30, and antenna 40 in a transmitchannel. The transceiver also includes the antenna 40, duplexer 30, alow-noise amplifier 50 and a receive channel downconverter or RXdownconverter 60 in a receive channel. A TX baseband signal containinginformation to be transmitted by the transceiver is upconverted from abaseband frequency to a RF frequency by the TX upconverter 10 beforebeing amplified by the power amplifier 20. The frequency modified TXbaseband signal is a RF transmit signal. The RF transmit signal, labeledTX signal and illustrated with arrows pointing toward the antenna 40, isamplified by the power amplifier 20 and coupled to the antenna 40 viathe duplexer 30. A remotely generated receive signal, labeled RX signaland illustrated with arrows pointing to the right side of the figure, isreceived by the antenna 40 and coupled via the duplexer 30 to thelow-noise amplifier 50. The low-noise amplifier 50 amplifies the RXsignal and forwards the amplified RX signal to the RX down converter 60and other components in the handset for further baseband processing.When the portable transceiver operates in a full duplex mode, theduplexer 30 is simultaneously processing the RF transmit signal and theRF receive signal. The duplexer 30 is an imperfect device and provideslimited isolation in the absence of a RX bandpass filter (e.g., a SAWfilter), hence some portion of the transmit signal energy, labeled TXleakage, is coupled into the receive path of the transceiver.

The plot in FIG. 1 shows the relative amplitude and frequencyrelationships of the desired RX signal and the TX leakage signal presentafter RX down conversion. Even though the TX leakage signal is shiftedin frequency from the desired RX signal, a TX IM2 signal is present inthe same range of frequencies as the RX signal. The TX IM2 signalresults from a second order non-linearity in the receive path of thetransceiver.

Therefore, it would be desirable to develop a transceiver architectureabsent SAW filters that is not adversely affected by the above describedinter-modulation interference.

SUMMARY

An embodiment of a transmit leakage signal canceller and method forcancelling second-order inter-modulation due to a transmit leakagesignal in a receive channel in a mobile handset includes a delayestimator and a digital signal adjuster. The delay estimator generatesan estimate of the delay between a transmit reference signal and thesecond-order inter-modulation due to the transmit leakage signal in areceive channel. The digital signal adjuster removes the estimate of thetransmit leakage second-order inter-modulation from a received signalbefore forwarding a modified signal to a baseband subsystem of theportable handset.

An embodiment of method for cancelling second-order inter-modulation dueto a transmit leakage signal in a mobile handset includes the steps ofinserting a canceller between a RF subsystem and a baseband subsystem ofthe mobile handset, using the canceller to generate an estimate of thesecond-order inter-modulation expected in a receive channel due toleakage of a transmit signal into the receive channel and combining asignal from the receive channel with the estimate of second-orderinter-modulation due to the transmit leakage signal.

The figures and detailed description that follow are not exhaustive. Thedisclosed embodiments are illustrated and described to enable one ofordinary skill to make and use the transmit leakage signal canceller andmethod for reducing second-order inter-modulation due to a transmitleakage signal in a receive channel of a mobile handset. Otherembodiments, features and advantages of the canceller and method will beor will become apparent to those skilled in the art upon examination ofthe following figures and detailed description. All such additionalembodiments, features and advantages are within the scope of thedisclosed systems and methods as defined in the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The transmit signal IM2 canceller and methods for cancelling IM2 in amobile handset can be better understood with reference to the followingfigures. The components within the figures are not necessarily to scale,emphasis instead being placed upon clearly illustrating the principlesand operation of the canceller and the methods. Moreover, in thefigures, like reference numerals designate corresponding partsthroughout the different views.

FIG. 1 is a schematic diagram illustrating the introduction of atransmitter leakage signal that generates IM2 on the desired receivesignal of a conventional full duplex transceiver.

FIG. 2 is a block diagram illustrating a simplified portable transceiverincluding a transmit signal IM2 canceller.

FIG. 3 is a schematic diagram illustrating an example embodiment of thetransmit signal IM2 canceller of FIG. 1.

FIG. 4 is a schematic diagram of an embodiment of the delay estimator ofFIG. 3.

FIG. 5 is a schematic diagram of an embodiment of the signal adjuster ofFIG. 3.

FIG. 6 is a flow chart illustrating an embodiment of a method forreducing second order inter-modulation in a mobile handset.

FIG. 7 is a flow chart illustrating an alternative embodiment of amethod for reducing second order inter-modulation in a mobile handset.

DETAILED DESCRIPTION

Although described with particular reference to a portable transceiveroperating under the wideband code division multiple access (WCDMA)modulation scheme, the canceller and method for reducing second orderinter-modulation in a mobile handset can be implemented in anycommunication device where full duplex operation of a transceiver isdesired. That is, the canceller and method for reducing second orderinter-modulation can be integrated with any full duplex communicationsystem in which leakage from a transmitter into a receiver results ininterference in a receive frequency band of interest.

The IM2 canceller and methods for cancelling or reducing IM2 in a mobilehandset can be implemented in hardware, software, or a combination ofhardware and software. When implemented in hardware, the IM2 cancellerand methods can be implemented using specialized hardware elements andlogic. When the IM2 canceller and methods are implemented partially insoftware, the software portion can be used to control one or morecomponents in the mobile handset so that various operating aspects canbe software-controlled. The software can be stored in a memory andexecuted by a suitable instruction execution system (microprocessor).The hardware implementation of the IM2 canceller and methods forcancelling or reducing IM2 in a mobile handset can include any or acombination of the following technologies, which are all well known inthe art: discrete electronic components, a discrete logic circuit(s)having logic gates for implementing logic functions upon data signals,an application specific integrated circuit having appropriate logicgates, a programmable gate array(s) (PGA), a field programmable gatearray (FPGA), etc.

The software for the IM2 canceller and methods for cancelling orreducing IM2 in a mobile handset comprise an ordered listing ofexecutable instructions for implementing logical functions, and can beembodied in any computer-readable medium for use by or in connectionwith an instruction execution system, apparatus, or device, such as acomputer-based system, processor-containing system, or other system thatcan fetch the instructions from the instruction execution system,apparatus, or device and execute the instructions.

In the context of this document, a “computer-readable medium” can be anymeans that can contain, store, communicate, propagate, or transport theprogram for use by or in connection with the instruction executionsystem, apparatus, or device. The computer readable medium can be, forexample but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, device,or propagation medium. More specific examples (a non-exhaustive list) ofthe computer-readable medium would include the following: an electricalconnection (electronic) having one or more wires, a portable computerdiskette (magnetic), a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flash memory)(magnetic), an optical fiber (optical), and a portable compact discread-only memory (CDROM) (optical). Note that the computer-readablemedium could even be paper or another suitable medium upon which theprogram is printed, as the program can be electronically captured, viafor instance optical scanning of the paper or other medium, thencompiled, interpreted or otherwise processed in a suitable manner ifnecessary, and then stored in a computer memory.

FIG. 2 is a block diagram illustrating a simplified portable transceiver100 including a TX IM2 canceller 200. The portable transceiver 100includes an input/output (I/O) element 102 coupled to a basebandsubsystem 110 via connection 104. The I/O element 102 represents anyinterface with which a user may interact with the portable communicationdevice 100. For example, the I/O element 102 may include a speaker, adisplay, a keyboard, a microphone, a trackball, a thumbwheel, or anyother user-interface element. A power source 142, which may be a directcurrent (DC) battery or other power source, is also connected to thebaseband subsystem 110 via connection 144 to provide power to theportable transceiver 100. In a particular embodiment, portabletransceiver 100 can be, for example but not limited to, a portabletelecommunication device such as a mobile cellular-type telephone.

The baseband subsystem 110 includes microprocessor (μP) 120, memory 122,analog circuitry 124, and digital signal processor (DSP) 126 incommunication via bus 128. Bus 128, although shown as a single bus, maybe implemented using multiple busses connected as necessary among thesubsystems within baseband subsystem 110.

Depending on the manner in which the TX IM2 canceller 200 and methodsfor cancelling IM2 are implemented, the baseband subsystem 110 may alsoinclude one or more of an application specific integrated circuit(ASIC), a field programmable gate array (FPGA), or any otherimplementation-specific or general processor. In an alternativeembodiment (not shown), one or more of an ASIC, a FPGA or otherimplementation-specific or general processor may be integrated in the RFsubsystem 130 to control the TX IM2 canceller 200 and related elementsfor cancelling IM2 in the portable transceiver 100.

Microprocessor 120 and memory 122 provide the signal timing, processingand storage functions for portable transceiver 100. Analog circuitry 124provides the analog processing functions for the signals within basebandsubsystem 110. The baseband subsystem 110 provides data and controlsignals to a radio frequency (RF) subsystem 130. The RF subsystem 130receives data from remote transmitters and forwards the received data tothe baseband system 110 for further processing. The RF subsystem 130includes a direct launch transmitter 150, a receiver 170, a poweramplifier controller 190, and the TX IM2 canceller 200. The elementswithin the RF subsystem 130 can be controlled by signals from thebaseband subsystem 110, which is connected to various baseband elementsvia bus 128. The elements within the RF subsystem 130 can be implementedvia application specific integrated circuits. Alternatively, the directlaunch transmitter 150, receiver 170, power amplifier controller 190 andthe TX IM2 canceller 200 may be located on a single RF integratedcircuit (IC).

The baseband subsystem 110 generates various control signals, such as apower control signal, that are used to control the power amplifiercontroller 190 and the power amplifier 160, as known to those skilled inthe art. The control signals on connection 128 may originate from theDSP 126, the microprocessor 120, or from any other processor within thebaseband subsystem 110, and are supplied to a variety of connectionswithin the direct launch transmitter 150, receiver 170 and poweramplifier controller 190. Additional control signals on connection 128originating from the DSP 126, the microprocessor 120, or from any otherprocessor within the baseband subsystem 110 are supplied to the TX IM2filter chain 155, the RX baseband processing chain 175 and the TX IM2canceller 200. It should be noted that, for simplicity, only the basiccomponents of the portable transceiver 100 are illustrated herein. Thecontrol signals provided by the baseband subsystem 110 control thevarious components within the portable transceiver 100. Further, thefunctions within the direct launch transmitter 150 and the receiver 170may be integrated into a transceiver.

The power amplifier controller 190 generates a power amplifier (PA)power control signal. The power control signal is coupled to the poweramplifier 160 via connection 195. The power control signal controls thepower output of the power amplifier 160 based on various inputs. Forexample, in an embodiment, a closed power control loop may influence thepower output of the power amplifier 160. In another embodiment, an openpower control loop may influence the power output of the power amplifier160. For example, in an embodiment, a signal received by a base stationwith which the portable transceiver 100 is communicating may issue apower control signal. In other embodiments, the baseband subsystem 110provides enable, standby and power control signals to the poweramplifier controller 190. In turn, the power amplifier controller 190processes the enable, standby and power control signals and generates apower control signal that is communicated to the power amplifier 160 onconnection 195.

If portions of the TX IM2 canceller 200 and the methods for cancellingIM2 in a mobile handset such as the portable transceiver 100 areimplemented in software that is executed by the microprocessor 120, thememory 122 will include IM2 canceller software 205. The TX IM2 cancellersoftware 205 comprises one or more executable code segments that can bestored in the memory 122 and executed in the microprocessor 120.Alternatively, the functionality of the TX IM2 canceller software 205can be coded into an ASIC (not shown) or can be executed by an FPGA (notshown), or another device or may be integrated into the RF subsystem 130of the portable transceiver 100. Because the memory 122 can berewritable and because a FPGA is reprogrammable, updates to the TX IM2canceller software 205 can be remotely sent to and saved in the portabletransceiver 100 when implemented using either of these methodologies.

The simplified portable transceiver 100 includes an embodiment of adirect launch transmitter 150, which includes an I/Q generator 136, a RFupconverter 152, a TX IM2 filter chain 155, and a power amplifier 160.The I/Q generator 136 converts digital information (i.e., the Cartesianin-phase (I) and quadrature (Q) data signal components or the digitaldata bit stream) within baseband subsystem 110 into a signal suitablefor further processing by the RF upconverter 152. The RF upconverter 152performs a digital to analog conversion and provides analog filteringbefore upconverting and forwarding the information to the poweramplifier 160 for amplification. The I/Q generator 136 and RFupconverter 152 generate a RF transmit signal. The TX IM2 filter chain155, receives the digital I/Q data from the I/Q generator 136 viaconnection 140, generates a digital transmit IM2 signal and forwards afiltered version of the digitally generated transmit IM2 signal to theTX IM2 canceller 200 via connection 157. The TX IM2 canceller 200receives a receive signal from the receiver 170 via connection 177 thathas been filtered by the RX baseband processing chain 175.

The TX IM2 filter chain 155 is a simplified version of the receivechannel filter processing performed in RX baseband processing chain 175.The TX IM2 canceller 200 generates an estimate of the transmit IM2 inthe receive channel introduced by the transmit signal and adaptivelyfilters the estimate before combining the estimate with the receivesignal to reduce or effectively cancel the transmit IM2 from the desiredreceive signal. In the illustrated embodiment, the TX IM2 canceller 200forwards a composite receive signal that includes the I data and Q datacomponents to the baseband subsystem 110. In alternative embodiments,the composite receive signal may be forwarded to elements withinreceiver 170 to separate the I data and Q data components prior tocommunicating the same to the baseband subsystem 110.

The RF upconverter 152 of the direct launch transmitter 150 combines andtransforms the digital I data and digital Q data signals to an analogsignal. In addition, the RF upconverter 152 upconverts the analog signalfrom a baseband frequency to an appropriate transmit frequency andprovides the upconverted analog signal at the transmit frequency to thepower amplifier 160 via connection 155. The power amplifier 160amplifies the transmit signal to an appropriate power level for thesystem given present conditions under which the portable transceiver 100is operating.

The I and Q components may take different forms and be formatteddifferently depending upon the communication standard being employed.For example, when the power amplifier 160 is used in aconstant-amplitude, phase (or frequency) modulation application such asthe global system for mobile communications (GSM), the phase modulatedinformation is provided by a modulator within the direct launchtransmitter 150. When the power amplifier 160 is used in an applicationrequiring both phase and amplitude modulation such as, for example,extended data rates for GSM evolution, referred to as EDGE or WCDMA, theCartesian in-phase (I) and quadrature (Q) components contain bothamplitude and phase information.

The power amplifier 160 supplies the amplified signal via connection 161to a front end module 162. The front end module 162 comprises an antennasystem interface that may include, for example, a duplexer having afilter pair that allows simultaneous passage of both transmit signalsand receive signals, as known to those having ordinary skill in the art.The transmit signal is supplied from the front end module 162 to theantenna 165.

A signal received by an antenna 165 is directed from the front endmodule 162 to the receiver 170. The receiver 170 includes variouscomponents to downconvert, filter, demodulate and recover a data signalfrom a received signal, as known to those skilled in the art. Ifimplemented using a direct conversion receiver (DCR), the receiver 170converts the received signal from an RF level to a baseband level (DC),or a near-baseband level (˜100 kHz). Alternatively, the received RFsignal may be downconverted to an intermediate frequency (IF) signal,depending on the system architecture. In the illustrated embodiment, thereceiver includes RF downconverter 172 and RX baseband processing chain175. The RF downconverter 172 receives the RF signal on connection 163from the front end module 162. The downconverted receive signal isforwarded on connection 173 to the RX baseband processing chain 175. TheRX baseband processing chain 175 processes the downconverted receivesignal and provides a filtered receive signal suitable for furtherprocessing by the TX IM2 canceller 200 before being forwarded to thebaseband subsystem 110. When the TX IM2 canceller 200 is active, therecovered receive signal information with IM2 cancelled from therecovered receive signal is supplied via bus 128 for further processingin the baseband subsystem 110.

FIG. 3 is a schematic diagram illustrating an example embodiment of eachof the RX baseband processing chain 175, the TX IM2 canceller 200 andthe TX IM2 filter chain 155 of FIG. 1. The RX baseband processing chain175 receives a downconverted representation of the receive signal fromthe RF downconverter 172 via connection 173 and generates a filtereddigital representation of the receive signal. As explained, the receivesignal provided by the RX baseband processing chain 175 on connection177 includes transmit channel induced IM2 due to leakage of thetransmitted signal into the receiver 170 (FIG. 2).

The RX baseband processing chain 175 includes an analog filter 302,delta-sigma analog-to-digital converter (ADC) 304, decimator 306,high-pass filter 308, equalizer element 310, droop correction element312 and a root-raised cosine (RRC) filter 314. The analog filter 302receives the downconverted receive signal and filters the same beforeforwarding the filtered receive signal to the delta-sigmaanalog-to-digital converter 304. The delta-sigma ADC 304 samples thefiltered receive signal and forwards a digital representation of thefiltered receive signal to the decimator 306. The decimator 306eliminates a select number of the digital samples provided to thedecimator 306 by the delta-sigma ADC 304. Thereafter, the high-passfilter 308 removes DC and very low frequency information in thedecimated samples and forwards the high-pass filtered receive data tothe equalizer 310, which alters the relative signal strengths ofdifferent frequencies in the filtered receive signal before forwardingthe equalized and filtered receive signal to the droop correctionelement 312. The droop correction element 312 further equalizes oradjusts the relative signal strengths of different frequencies in thefiltered receive signal before forwarding the droop corrected receivesignal to the RRC filter 314. The RRC filter 314 is a pulse-shapingfilter. The RRC filter 314 is configured to eliminate interchannelinterference in a symbol stream. The RRC filter 314 has an impulseresponse that is zero at nT (where n is an integer and T is the sampleperiod, except where n=0. As indicated in FIG. 3, the RRC filteredsymbol stream representing the receive signal is forwarded viaconnection 177 to the TX IM 2 canceller 200.

The TX IM2 filter chain 155 includes an I*I+Q*Q combiner or combiner322, scaler element 324, digital filter 326, high-pass filter 328,equalizer element 330, droop correction element 332 and RRC filter 334.The combiner 322 generates a digital symbol based on the real andimaginary components of each of the I data signal and the Q data signalreceived via connection 140. The scaler 324 receives the digital symbolsfrom the combiner 322 and adjusts the symbols before forwarding thesymbols to the digital filter 326. The digital filter 326 samples andadjusts the scaled symbols before forwarding the same to the high-passfilter 328. Thereafter, the high-pass filter 328 removes DC and very lowfrequency information in the samples and forwards the high-pass filteredtransmit data to the equalizer 330, which alters the relative signalstrengths of different frequencies in the filtered transmit signalbefore forwarding the equalized and filtered transmit signal to thedroop correction element 332. The droop correction element 332 furtherequalizes or adjusts the relative signal strengths of differentfrequencies in the filtered transmit signal before forwarding the droopcorrected transmit signal to the RRC filter 334. The RRC filter 334,which is similar to the RRC filter 314 in the RX baseband processingchain 175, reshapes the TX IM2 signal and eliminates inter-channelinterference in the transmit symbol stream. The RRC filtered symbolstream representing an estimate of IM2 due to the transmit signal in thereceive channel is forwarded via connection 157 to the TX IM 2 canceller200.

The TX IM2 canceller 200 includes a delay estimator 400 and a signaladjuster 500. The delay estimator 400 receives the processed stream ofdigital symbols from the RX baseband processing chain 175 on connection177 and the stream of digital symbols representing the estimate of theTX IM2 as generated by the TX IM2 filter chain 155 on connection 157.The processed stream of digital symbols from the RX baseband processingchain 175 includes both the desired data signal and the transmitterinduced TX IM2. The TX IM2 canceller aligns the received symbol streamsin a delay estimator 400. The delay estimator 400 selects an appropriatetransmit data symbol from the stream of processed symbols provided onconnection 157 and forwards the select symbol to the signal adjuster 500on connection 405. In turn, the signal adjuster 500 removes theestimated TX IM2 from the desired data signal and forwards a modifiedreceive signal to the baseband subsystem 100 via bus 128.

FIG. 4 is a schematic diagram of an embodiment of the delay estimator400 of FIG. 3. The delay estimator 400 is a three-tap filter. A firsttap includes a delay element 410, a multiplier 412 and an accumulator414. The delay element 410 receives the estimated TX IM2 symbols viaconnection 157 and forwards the same after delay Ts via connection 415to a second tap and to the select element 450. The multiplier 412combines the estimated TX IM2 symbol with the receive data symbol onconnection 177. The multiplier 412 forwards a combined first tap signalto the accumulator 414, which determines a peak magnitude of thecombined first tap signal. The peak magnitude of the first tap signal isforwarded to maximum detector (MAX.) 440.

A second tap includes a delay element 420, a multiplier 422, and anaccumulator 424. The delay element 420 receives the estimated TX IM2symbols via connection 415 and forwards the same after delay Ts viaconnection 425 to a third tap and to the select element 450. Themultiplier 422 combines the estimated TX IM2 symbol with the receivedata symbol on connection 177. The multiplier 422 forwards a combinedsecond tap signal to the accumulator 424, which determines a peakmagnitude of the combined second tap signal. The peak magnitude of thesecond tap signal is forwarded to maximum detector 440.

A third tap includes a delay element 430, a multiplier 432, and anaccumulator 434. The delay element 430 receives the estimated TX IM2symbols via connection 425 and forwards the same after delay Ts viaconnection 435 to the select element 450. The multiplier 432 combinesthe estimated TX IM2 symbol with the receive data symbol on connection177. The multiplier 432 forwards a delayed output at the third tapsignal to the accumulator 434, which determines a peak magnitude of thecombined third tap signal. The peak magnitude of the third tap signal isforwarded to maximum detector 440. The maximum detector 440 determineswhich of the three detected peak magnitudes is the greatest and forwardsa signal via connection 445 to the select element 450 identifying thesame. In turn, the select element 450 forwards the identified one of theestimated TX IM2 symbols from one of the first tap, the second tap orthe third tap on connection 405 gain correction in the signal adjuster500. The delay between the TX IM2 filter chain 155 and the RX basebandprocessing chain 175 is fixed for a certain design and the delayestimator 400 could be used once during a factory calibration or duringa power up sequence to find an optimum delay path among connection 415,connection 425, and connection 435.

FIG. 5 is a schematic diagram of an embodiment of the signal adjuster500 of FIG. 3. The signal adjuster 500 receives the processed receivesignal on connection 177 and the estimate of the TX IM2 present in theprocessed receive signal on connection 405. The estimate of the TX IM2is labeled “TX SIGNAL REFERENCE” in FIG. 5. The signal adjuster 500includes an adaptive filter 510, a combiner 520, a delay element 530 anda least mean square element 540. The adaptive filter 510 receives theestimate of TX IM2 on connection 405 and provides a filtered version ofthe estimate, Y_(i), on connection 515. The combiner 520 receives theprocessed receive signal on connection 177 and applies the same at apositive or additive input. The combiner 520 receives the adaptivelyfiltered version of the estimate of TX IM2 at a negative or subtractiveinput. The result of the mathematical combination of the two signals isa representation of the desired receive data symbol. The desired receivedata symbol, e_(i), is forwarded via bus 128 to both the basebandsubsystem 110 and via a feedback path to the LMS element 540. Thefeedback path includes the delay element 530 and the least mean squareelement 540. The least mean square element 540 receives the TX signal orreference signal on connection 405. The combination of the delay element530 and the least mean square element 540 generates a filtercoefficient, W_(i), which sets the characteristics of the adaptivefilter 510 that is applied to the estimate of the TX IM2 received onconnection 405.

As indicated in FIG. 5, the processed RX signal on connection 177 isrepresented by the following relationship:

D _(i) =S(t _(i))+IM2(t _(i))  Eq. 1

The estimate of the TX IM2 present in the receive signal is representedby the relationship in equation 2 below.

X _(i)=I{tilde over (M)}2(t _(i))  Eq. 2

The output of the adaptive filter is represented by the relationship inequation 3 below.

Y_(i)=X_(i) ^(T)W_(i)  Eq. 3

where, e_(i)=D_(i)−Y_(i) and W_(i+1)=W_(i)+2μe_(i)X_(i).

The filter coefficients, W_(i), are adjusted to minimize the differencebetween the TX IM2 component carried in the RX signal on connection 177and the reference TX IM2 signal on connection 405 from the delayestimator 400. The relationship that defines how the coefficients areupdated includes a factor, μ that defines the rate of update. The rateof update of the coefficients controls the convergence and stability ofthe signal adjuster 500. The combination of the RX baseband processingchain 175, the TX IM2 filter chain 155 and the TX IM2 canceller 200reduces the TX IM2 to an acceptable level in the received signal. Thatis, the TX IM2 canceller 200 minimizes the amount of IM2 in the RXsignal on connection 177 that results from transmitter leakage in thefront end module 162.

The flow diagrams of FIGS. 6 and 7 show the architecture, functionality,and operation of possible implementations via software and or firmwareassociated with an IM2 canceller arranged with similar transmit andreceive channel filter chains in a mobile handset. In this regard, ablock can represent a module, segment, or portion of code, whichcomprises one or more executable instructions for implementing thespecified function(s). When the IM2 canceller is implemented viahardware, hardware and firmware or a combination of hardware andsoftware, one or more blocks in the flow diagram may represent a circuitor circuits. Alternatively, the described functions can be embodied insource code including human-readable statements written in a programminglanguage or machine code that comprises instructions recognizable by asuitable execution system such as a processor in a computer system. Themachine code may be converted from the source code, etc.

FIG. 6 is a flow chart illustrating an embodiment of a method forreducing second order inter-modulation in a mobile handset. The method600 begins with block 602 where a canceller is inserted between a radiofrequency subsystem and a baseband subsystem of a mobile handset.Thereafter, as indicated in block 604, the canceller generates anestimate of the second order inter-modulation expected in a receivechannel of the mobile handset due to leakage of a transmit signal intothe receive channel. As shown in block 606, a signal from the receivechannel is combined with the estimate of the second orderinter-modulation to reduce the second order inter-modulation in thereceive signal.

FIG. 7 is a flow chart illustrating an alternative embodiment of amethod for cancelling or reducing IM2 in a mobile handset. The method700 begins with block 702 where a canceller is inserted between a radiofrequency subsystem and a baseband subsystem of a mobile handset. Asfurther indicated in block 702, the canceller includes a delay estimatorand a signal adjuster. In block 704, a first signal in a receive channelis processed by one or more of high-pass filtering, equalizing, droopcorrecting and root-raised cosine filtering. In block 706, a secondsignal from a transmit channel is similarly processed by one or more ofhigh-pass filtering, equalizing, droop correcting and root-raised cosinefiltering. Thereafter, as indicated in block 708, the canceller appliesthe first and second signals at respective inputs of the delay estimatorto generate an estimate of the delay between a reference IM2 (i.e., thesecond signal from the transmit channel) and IM2 in the desired signal(i.e., the first signal from the receive channel) Next, as indicated inblock 710, the estimate is applied at an input of the signal adjuster.The signal adjuster adaptively filters and combines the filteredestimate with a signal from the receive channel. As shown in FIG. 5, thecombination is a subtraction of the estimated IM2 in the receive channeldue to transmit signal leakage into the receive channel. In block 712,the signal adjuster determines a least mean square error in a feedbackpath to minimize the interference power in the received signal.

While various embodiments of the IM2 canceller, delay estimator, signaladjuster and methods for cancelling IM2 in a mobile handset have beendescribed, it will be apparent to those of ordinary skill in the artthat many more embodiments and implementations are possible that arewithin the scope of this disclosure. Accordingly, IM2 canceller, delayestimator, signal adjuster and methods for cancelling IM2 are not to berestricted except in light of the attached claims and their equivalents.

1. A digital transmit-signal IM2 canceller for a portable handset thatoperates in a full-duplex mode, the digital transmit-signal canceller,comprising: a delay estimator arranged to receive a first input from areceive channel and a second input from a transmit channel of thehandset and generates an estimate of the second-order inter-modulationthat the transmit channel introduces in the receive channel; and adigital signal adjuster arranged to receive and remove the estimate ofthe second-order inter-modulation from the receive channel beforeforwarding a modified signal to a baseband subsystem of the portablehandset.
 2. The digital transmit-signal canceller of claim 1, whereinthe delay estimator comprises a N-tap digital filter.
 3. The digitaltransmit-signal canceller of claim 2, wherein N is
 3. 4. The digitaltransmit-signal canceller of claim 1, wherein the first input and thesecond input have been processed by respective filter chains.
 5. Thedigital transmit-signal canceller of claim 4, wherein the respectivefilter chains comprise a series combination of a high-pass filter, anequalizer, a droop correction element and a root-raised cosine filter.6. The digital transmit-signal canceller of claim 1, wherein the digitalsignal adjuster comprises an adaptive filter, a combiner, and a feedbackpath.
 7. The digital transmit-signal canceller of claim 6, wherein thecombiner subtracts the output of the adaptive filter from a receivesignal.
 8. The digital transmit-signal canceller of claim 6, wherein thefeedback path comprises a delay element and a least mean square element.9. The digital transmit-signal canceller of claim 8, whereincoefficients are adjusted to minimize the difference between a signalfrom the receive channel and a reference estimate of the second-orderinter-modulation.
 10. A method for cancelling second-orderinter-modulation in a mobile handset transceiver, comprising: insertinga canceller between a RF subsystem and a baseband subsystem of themobile handset; using the canceller to generate an estimate of thesecond-order inter-modulation expected in a receive channel due toleakage of a transmit signal into the receive channel; and combining asignal from the receive channel with the estimate of the second-orderinter-modulation.
 11. The method of claim 10, wherein inserting acanceller between a RF subsystem and a baseband subsystem of the mobilehandset comprises inserting a delay estimator and a signal adjuster. 12.The method of claim 11, wherein inserting a delay estimator and a signaladjuster comprises applying a N-tap digital filter.
 13. The method ofclaim 10, wherein using the canceller comprises receiving a first signalat a first input of the canceller from the receive channel and receivinga second signal at a second input of the canceller from a transmitchannel.
 14. The method of claim 13, wherein receiving a first signaland receiving a second signal further comprises receiving signals thathave been processed by respective filter chains.
 15. The method of claim14, wherein receiving a first signal and receiving a second signalfurther comprises high-pass filtering, equalizing, droop correcting androot-raised cosine filtering.
 16. The method of claim 11, whereininserting a delay estimator and a signal adjuster further comprisesadaptive filtering and combining.
 17. The method of claim 16, whereincombining comprises subtracting a result of the adaptive filtering fromthe first signal.
 18. The method of claim 16, wherein adaptive filteringcomprises applying coefficients to minimize a difference between asignal from the receive channel and a reference estimate of thesecond-order inter-modulation or other interferences generated fromtransmit signal.
 19. The method of claim 11, wherein inserting a delayestimator and a signal adjuster further comprises delaying andcalculating a least mean square error in a feedback path.
 20. The methodof claim 19, wherein calculating a least mean square error in a feedbackpath comprises adjusting coefficients.